Fuel cells, in which hydrogen and oxygen combine in an electrochemical reaction to generate electricity with by-product water, have emerged as an alternative to the conventional power generation methods such as internal combustion engines and the like with such obvious advantages as cleanliness, quietness, and efficiency. Fuel cells can find applications in many fields including portable power, transportation and stationary power plants. In general, a fuel cell is an electrochemical device that directly converts the chemical energy of a fuel/oxidizer mixture into electricity. The direct conversion of fuel into electricity means that fuel cells operate at higher efficiencies (˜50-65% based on the LHV of fuel) than conventional power generation systems that convert fuel into heat that produces mechanical work for electricity production. Conventional power generation systems are Carnot limited and lose efficiency because of thermodynamic and mechanical limitations in the system. Aside from efficiency considerations, fuel cells offer several other advantages over conventional power systems. In today's climate of increasing environmental awareness, fuel cell systems have the potential to substantially reduce air pollution associated with electricity production. For most types of fuel cells, the only by-product of electricity production is water if hydrogen is used as fuel. The higher system efficiencies for fuel cells translate into enhanced fuel utilization and therefore reduced CO2 emissions compared to lower efficiency systems. Fuel cell power plants will be capable of exceeding stringent present and future environmental regulations for particulates, NOx, and SOx emissions. In addition, the absence of moving parts in fuel cell mechanical systems greatly reduces the noise associated with conventional power plants, and fuel cell power plants have high reliability with low maintenance. Owing to its advantageous characteristics, amongst other things, fuel cells are particularly applicable in those areas requiring highly reliable, stand-alone power supplies such as is required in telecom and emergency stations.
Among low temperature fuel cells, the proton exchange membrane fuel cells (PEMFCS) have received considerable attention largely due to its nature of low temperature that leads to quick startup as being viewed important for electric vehicles. Since the electrolyte is a polymeric material, there is no free corrosive liquid inside the cell (water is the only liquid), hence material corrosion is kept to a minimum. In addition, PEMFCs are simple to fabricate and have demonstrated a long life.
A single fuel cell consists of an anode and a cathode separated by an electrically insulating electrolyte, which in the case of PEM fuel cells is the proton exchange membrane. To promote the desired electrochemical reactions, the catalyst layer is formed on the surface of the PEM to form a porous electrode membrane assembly (MEA). A hydrogen rich fuel (or pure hydrogen) permeates the porous electrode material of the anode and reacts with the catalyst layer to form hydrogen ions and electrons (H2→2H++2e−). The hydrogen ions migrate through the PEM to the cathode electrode, where the oxygen-containing gas supply (usually air) also permeates through the porous material and reacts with the hydrogen ions and electrons (which arrive from the anode through external circuitry) to produce water and heat (½O2+2H++2e−→H2O+Heat). A practical individual fuel cell generally consists of an electrically conductive anode plate with certain types of flow channels, an MEA and gas diffusion layer (GDL) (or the two integrated), and an electrically conductive cathode plate with certain types of flow channels as well as sealing materials between MEA and the plates. A single cell generally provides about 0.6-0.8 volts at a current density on the order of a few hundred mA/cm2, therefore, a number of fuel cells need to be stacked together to achieve desired electrical power output. The stacked multiple fuel cells are packed between two endplates typically with the attachment means such as tie rods.
A fuel cell power system, as schematically shown in FIG. 1, is centered with a fuel cell stack having two endplates (21,22) on which are positioned fluid connectors for receiving and exhausting fuel, oxidant and coolant. Hydrogen or hydrogen rich fuel (100) from a fuel processor is supplied to the fuel cell stack fuel inlet, with a check valve (101) usually installed on the supply line to prevent any possible backflow from the stack. The depleted fuel from the stack can be either recycled back to the inlet by an appropriate means such as an injector (103) or be sent back to an auxiliary burner to produce heat or to a burner incorporated with the fuel processor to supply heat for fuel reforming. A valve (102) may be installed at the fuel exhaust line to maintain the stack at an appropriate pressure or control the fuel flow. Air is generally fed to the stack after filtration (209), compressed (200) and humidified (202). A practical and convenient air humidification method is the use of a humidifier (such as an enthalpy wheel or a fiber membrane) that exchanges humidity between saturated or even liquid water containing cathode exhausting air and relatively dry and cool incoming air (201). Depleted air, after giving moisture to the incoming air, is preferably passed through a condenser or water separator, or simply a drain valve (206) prior to the vent. To remove the heat released from the fuel cell reactions in order to keep the stack at a preferable operating temperature, a cooling loop is designed, which generally includes a coolant pump (310), a coolant filter (301), a heat recover heat exchanger (sometimes called cogeneration heat exchanger) (304), a backup heat exchanger (sometimes called radiator) (306) that is used to dissipate the heat to the environment only when there is no sufficient cogeneration, and a coolant storage tank (308). The two heat exchangers (304) and (306) can be reduced to one if the fuel cell system is designed to be without cogeneration.
Conventional fuel cell systems are constructed with these multiple components being individually installed and connected together through pipelines and fittings. They are then housed in a package chamber. Such a package, for example, has been shown in FIG. 4 of U.S. Patent Application Publication 2003/0138688 A1, published on Jul. 24, 2003. It is commonly understood in the field that in order to reduce the system size and volume, all these functional components are tightly packaged inside the housing chamber, therefore resulting in increased complexity of the mechanical layout and difficulty in insulating individual components and pipelines and providing maintenance service due to limited accessibility as a result of space tightness.
There are recent efforts in the field to improve the system compactness by integration of multiple components. U.S. Patent Application Publication 2003/0148157 A1 and U.S. Pat. No. 6,605,378 B2, published Aug. 7, 2003 and Aug. 12, 2003, respectively, disclose an integrated assembly including an enthalpy recovery device for transferring moisture and heat from fuel cell cathode exhaust and burner exhaust to incoming cathode air, a water reservoir and a degasifier. All of these are functionally integrated and housed in a chamber. The housing chamber does not include heat exchangers for cogeneration, air compressor/blower for supplying air to cathode, and other accessories as commonly involved in fuel cell power systems as described in FIG. 1.
There is a need for an integration of multiple components of fuel cell systems including fuel cell stacks and associated heat exchangers and other accessories such that weight, volume, and complexity are reduced.